1. Field of the Invention
The present invention relates generally to rotating disk technology, particularly to disk drive apparatus employed as a memory device and, more specificaly, to a technique for determining computer hard disk alignment and alignment shifts and for providing compensation.
2. Description of the Related Art
In the state of the art of digital data storage, hard disks form the basis for main memory in most computer systems. Hard disk drives provide rapid access to a very large number of data records. Magnetic storage hard disk drives are faster than floppy disk, magnetic tape, and optical disk drives, and cost less than equivalent capacity semiconductor memory. Fundamentals of magnetic recording and disk drives can be found in The Complete Handbook of Magnetic Recording by Finn Jorgensen, copyright 1988 (3rd Edition, TAB BOOK Inc., Blue Ridge Summit, Pa.)
FIG. 1 (Prior Art) illustrates the fundamental components of a computer memory, hard disk apparatus (generally referred to simply as the "hard drive"), head-disk assembly 101. A stack 103 of individual disks 103a, each having magnetic recording surface layers (on both sides of the disk), rotates (arrow 105) at a constant speed (e.g., approximately 5400 RPM) on a hub 106 suitably mounted on a base plate 108. Mounted in a head stack assembly 111, load beams 107 extend magnetic transducers ("heads") 109 which are selectively positioned with respect to each disk's recording surface in order to read and write digital data on the disks 103a (see also FIG. 2). A coil 113 on the head stack assembly 111 interacts with a magnetic drive subassembly 117 to swing the head stack assembly via a rotary shaft, or pivot bearing, 115 reversibly driven about the shaft axis, selectively moving the heads 109 from the inner diameter data tracks to the outer diameter data tracks of the recording surfaces. Appropriate electronics (not shown) are provided for controlling positioning and performing read/write functions via the heads 109. (See also FIG. 2 regarding disk formatting). The head stack assembly 111 in combination with the magnetic drive subassembly 117 and pivot bearing 115 is referred to as the "actuator assembly." In the state of the art, the typical read/write head may have physical dimensions of approximately 0.080-inch by 0.00125-inch; the height of the head above the disk surface may be on the order of approximately 0.75-micro-inch. Because of its shape, the head stack assembly main body part which encompasses the rotary shaft 115 is referred to as an "E-block." A unitary E-block assembly for use in a disk drive is disclosed in U.S. Pat. No. 5,095,396 (Putnam et al., assigned to the common assignee of the present invention.)
FIG. 2 (Prior Art) illustrates the fundamental constructs (dimensions exaggerated) involved in recording data on a magnetic disk 203. Physically, a hard drive disk comprises an aluminum substrate platter having a series of thin film layers thereon used in the data recording process and an outer, protective lubricant layer open to the environment (see e.g., U.S. Pat. No. Re. 32,464 (Aine) and its related patents). The disk 203 is formatted by dividing it into sectors 212 by a number of radially-extending spokes 214 placed at regular angular intervals about the disk. The spokes 214 are areas on the disk containing tracking servo bursts and sector identification information. The sectors 212 are areas on the disk containing data blocks 216, each block having a fixed amount of data, for example 512-bytes. The data blocks 216 occupy circular tracks 218. The tracks 218 are grouped into bands 220; all of the tracks 218 in a given band 220 contain the same number of radially aligned data blocks 216. The section of a band 220 where a set of radially aligned data blocks 16 is recorded is called a block frame 222. The number of block frames 222 per band increases with band 220 radius. The beginning and end of a block frame 222 are defined by a timing system in a disk controller (not shown). The number of bands 220 is maximized if each band has exactly one more block frame 222 than its inwardly adjacent band. In that case, the number of bands 220 is simply the difference between the number of block frames 222 in the outermost band and the number of block frames in the innermost band. The spokes 214 are numbered, e.g., zero to seven, in the direction opposite to disk rotation. The sectors 212 are also numbered, each sector being numbered the same as the immediately preceding spoke 214. The block frames 222 in each band 220 are numbered starting from zero; block frame zero in each band is adjacent to spoke zero. In order to maximize storage capacity of the disk 210, the data blocks 216 along the innermost track 218 of each band 220 are recorded as close as possible to a predetermined maximum linear bit density. As a result, the data rate in megabytes per second in each band increases with radius. The number of tracks 218 is predetermined for a given disk, normally spaced as close as possible to maximize storage density. In the state of the art, typical track density is approximately fifteen thousand (15,000) TPI; a typical data bit density is approximately two-hundred twenty thousand (220,000) BPI; average access time to find any particular data stream is approximately five-to nine milliseconds, holding several gigabytes of information.
The location of any recorded programs and data is stored in a directory area on a disk and informs the disk operating system about the exact sector and track number where the recorded data are to be found. Servo burst signals recorded in the spokes 214 provide positioning information; generally, amplitude and phase of servo-signals provide correction signals to the motor drive electronics associated with the actuator assembly. As known in the art, the design of servo-follower mechanisms, associated electronics, and optimized servo-tracking algorithms requires an analysis of the specific disk drive design implementation. While servo-tracking algorithms can provide for track following where small distortions are involved (e.g., .+-.100-microinch), inherent limitations in servo-tracking algorithms limits the tracking correction which is available for gross track shifts such as might occur if the head-to-track alignment is skewed, such as by a shock event. With the recognition of the advanced state of the art of such servo technology, a further detailed description is not necessary to an understanding of the present invention for a person skilled in the art.
As should now be recognized, considering the speed of the disk, the size of the heads, the complex data formatting on the disk, servo-follower limitations, and the high track and bit densities, for reliable read/write functionality it is critical that head-to-track alignment be precise.
Disk drive durability has to be maintained under sometimes severe environmental conditions, particularly during computer assembly and in the actual use of portable computers. One of the parameters that affect drive durability is shock, used to describe impact loading of drives. Shock is characterized by its magnitude in G-forces and shock duration. In disk drive technology, withstanding a shock implies that the head-media interface reliability is not compromised due to violent dynamic response of drive components following the shock event. To improve drive insensitivity to both computer assembly process, transportation, in-use handling conditions, and any other environmental situation in which a shock might be imparted to a unit, drive manufacturers are forced to design systems able to withstand higher G-forces over shorter durations; an exemplary shock load specification goal is for a disk drive to withstand a shock of 1000-G's at 1-millisecond without affecting performance.
One of the primary drive failure modes caused by a shock event is referred to as "disk shift" or "disk slippage." As shown in FIGS. 3A and 3B and discussed in more detail elsewhere in this specification, following a shock event, disk shift may have changed the head-to-track alignment. Due to a gross shift of written tracks of a formatted disk, the head is unable to follow during a read cycle, resulting in data read mode failures. There is a need for a method and apparatus for facilitating the determining and compensating of disk shifts.